1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device that is capable of maintaining a cell gap between an upper substrate and a lower substrate, and obtaining an air removing passage for exhausting air.
2. Description of the Related Art
In general, a liquid crystal display device is constructed of a pad unit and a display panel. The pad unit, which includes a driving circuit, transmits a signal to the display unit and the display panel transmits an image to a viewer corresponding to the signal. The display panel includes an upper substrate, a lower substrate and a liquid crystal, which is filled in between the upper and lower substrates.
FIG. 1 is an exploded perspective view showing the basic structure of a color liquid crystal display panel that uses a thin film transistor in each pixel. As shown in FIG. 1, a color filter 13 and a transparent common electrode 15 are formed on a lower side of the upper substrate 11, and a lower substrate 17 is positioned such that a specified interval or gap is between the upper substrate 11 and lower substrate 17. Subsequently, a liquid crystal (not shown) is filled between the two substrates.
The lower substrate 17 has an array of pixels that each include a switching device 21 and a transparent pixel electrode 29. The distinctiveness of an image displayed on a liquid crystal display panel is affected by the resolution, which is based upon the number and dimensions of the pixels formed in an array on the lower substrate 11. The dimensions of a pixel are designed according to the desired resolution of the liquid crystal display panel. A plurality of gate lines 25 and data lines 27, which cross-over each other in the a matrix form, are formed between the respective pixels. The pixel electrode 29 of a pixel applies an electric field across the liquid crystal between the upper substrate 11 and lower substrate 17 together with the common electrode 15. The thin film transistor 21 of a pixel is formed in the vicinity of where a gate line 25 and a data line 27 cross-over each other. The gate electrode (not shown) of the thin film transistor 21 connects to a gate line 25 and the source electrode (not shown) of the thin film transistor 21 connects to a data line 27.
The general fabrication process of a liquid crystal display panel includes forming an upper substrate with a common electrode on a color filter and a lower substrate with an array of pixels formed thereon. Then the upper and lower substrates are positioned such that the side of the upper substrate having the common electrode formed thereon faces the side of the lower substrate having the pixels formed thereon. Subsequently, a seal line is formed between the two substrates about the perimeter of the two substrates with an opening in the seal to be used as an injection port. Thereafter, a liquid crystal is injected between the upper substrate and the lower substrate, and the injection port is sealed, thereby completing the liquid crystal display panel. The light transmittance of the liquid crystal display panel is controlled by a voltage applied to each pixel electrode such that an image is displayed by controlling the liquid crystal to have a light shutter effect.
Fabrication of a liquid crystal display panel has the characteristics that, compared with the thin film transistor (TFT) process or the color filter process, there is no repeated step. The fabrication of a liquid crystal display panel can generally be divided into the steps of forming an orientation film for orienting liquid crystal, forming a cell gap and cutting a master panel into panels. The fabrication process of the liquid crystal display device will now be described with reference to FIG. 2, which depicts a flow chart for fabricating a liquid crystal display panel from a master panel. First, referring to the step ST1 in FIG. 2, a plurality of thin film transistors (TFTs) are arranged as switching devices on the lower substrate where pixel electrodes are then formed for each TFT. Next, as referred to in step ST2, an orientation film is formed on the lower substrate and processed. Forming the orientation film can include the coating of a copolymer thin film and the step of processing the orientation film can include the steps of rubbing the copolymer thin film.
Generally, the copolymer thin film is called an orientation film, which is typically coated with a uniform thickness on the entire lower substrate, and the rubbing is performed uniformly across the lower substrate. The process of rubbing determines an initial direction or arrangement of the liquid crystal. By rubbing the orientation film, the liquid crystal can be driven normally and uniform display characteristics can be obtained. In general, the orientation film is from the polyimide group or an organic substance. The process of rubbing is the act of rubbing the orientation film in a predetermined direction with a cloth such that the liquid crystal will subsequently align in the predetermined direction.
As referred to in step ST3 of FIG. 2, a seal pattern is formed enclosing designated areas of the master panel. The seal pattern can serve two functions. First, the seal pattern defines the panel of a liquid crystal display panel from the master panel as well as the gap of a panel. Secondly, the seal pattern prevents leakage of the injected liquid crystal from the panel unit. In the step ST3, the seal pattern is formed with a thermosetting resin deposited in a desired pattern using a screen printing technique.
Spacers with a predetermined size are sprayed or distributed, as referred to in step ST4 of FIG. 2, to maintain a uniform gap between the upper substrate and the lower substrate of a panel. Thus, the spacers should be sprayed or distributed with a uniform density across the lower substrate. Generally, the spacers are sprayed using a wet spray method in which a mixture of spacers and alcohol, or the like, is sprayed, or a dry spray method in which only the spacers are sprayed.
Subsequent to the spraying of spacers, the upper substrate and the lower substrate are attached, as referred to in step ST5 of FIG. 2. The precision in attaching the upper substrate and the lower substrate is such that the color filters are aligned with respective pixels is done within a error margin of a few microns. If the alignment of the two substrates is beyond the error margin, light leakage can degrade picture quality such that desired control of light transmission will not occur in driving of the liquid crystal display panel. Next, the master panel fabricated using the steps ST1-ST5 above is cut into panel units, as referred to in step ST6 of FIG. 2.
In an earlier fabrication process of a liquid crystal display panel that is different than the process described in steps ST1-ST5 above, liquid crystal was injected into the panels, defined by a seal pattern on the master panel, and then the panels were cut from the master panel into individual panels. Since the number of panels on a master panel and/or the size of the panels on the master panel have increased, the method described in steps ST1-ST5 is used to create the individual panels and then liquid crystal is injected into the individual panels. Cutting panels includes scribing a break line on the surface of the master panel with a pen made of a diamond or some other material having a hardness higher than that of the substrates, and applying a breaking force such that the substrates break along the break line. Then, as referred to in step ST7 of FIG. 2, liquid crystal is injected in each of the panels.
A liquid crystal display panel has a gap of a few micrometers between the upper and lower substrates over an area of hundreds of square centimeters. Therefore, a vacuum injection method using a pressure difference between the inside and the outside of the panel is widely used as a method for effectively injecting liquid crystal into such a panel.
Referring to the step ST5 described above, the uniformity of the thickness and height of the liquid crystal cell gap formed by the seal together with the spacers are critical factors that determine picture quality. However, the surface level of the lower substrate may not be uniform over the region where the seal is deposited, and thus the cell gap may not be not uniform in a liquid crystal panel. The occurrence of such a lack of uniformity in a liquid crystal cell gap of a related art liquid crystal display panel will now be described in detail with reference FIGS. 3, 4A, 4B and 4C and 5.
Typically, improved yields for liquid crystal display devices are sought by producing a plurality of liquid crystal display panels from a large-scale glass substrate 33 or master panel that is cut into panels such that a plurality of panels are formed simultaneously. Accordingly, seals 31 are printed on a large-scale glass substrate 33 to define a plurality of panels 32, as shown in FIG. 3. A seal 31 is printed along the periphery of each panel 32 formed on the large-scale glass substrate 33, and a liquid crystal injection port 35 is formed in the seal pattern 31 on one side of each panel 32.
A dummy seal pattern 37 is formed at a periphery of the glass substrate 33 and between the panels 32. The dummy seal pattern 37 serves as a support bar against a strong mechanical impact during a cutting step in which the large-scale glass substrate 33 is cut into panels, as described above with regard to step ST7 in FIG. 2. Thus, the dummy seal pattern prevents errant breakage of the upper substrate or the lower substrate other than along break lines on the substrates while maintaining the cell gap between the upper substrate and the lower substrate.
FIG. 3 depicts an enlarged portion of a panel 32 that includes an image display part 34 on which liquid crystal pixels are arranged in a matrix form, gate pads 38 connected to gate lines of the image display part 34 and data pads 36 connected to data lines. The gate pads 38 and the data pads 36 are formed at the respective peripheral regions of the lower substrate 17 which are not overlapped with the upper substrate 11 when the large-scale glass substrate 33 is broken into individual panels. The gate pads 38 supply a scan signal supplied from a gate driver integrated circuit to the gate lines of the image display part 34. The data pads 36 supply image information supplied from a data driver integrated circuit to the data line of the image display part 34.
As shown in the FIG. 3, the data lines from the data pads 36 on which the image information is supplied and gate lines from the gate pads 38 on which the scan signal is supplied are disposed to cross-over one another on the lower substrate of the image display part 34. Further, the lower substrate includes pixel cells between the gate and data lines that each have a thin film transistor for switching, a pixel electrode connected to the thin film transistor, and a passivation film formed over the pixel electrode and the thin film transistor of the pixel cells.
FIG. 3 does not show the color filters separately coated on the upper substrate for each pixel that are separated by a black matrix or the common electrode on all of the color filters. As stated above, the lower substrate and the upper substrate are separated by a cell gap that is filled with liquid crystal. The lower substrate and the upper substrate are attached by the seal 31 formed at the perimeter of the image display part 34, that has a data pad side 40a, a liquid crystal injection port side 40b, an open side 40c, and a gate pad side 40d. 
The liquid crystal cell gap between the lower substrate and the upper substrate will now be described in detail in reference to sectional views of seal attachment regions. FIG. 4A shows a sectional view of a seal attachment region for the gate pad side 40d taken along line I-I′ in FIG. 3. FIG. 4B shows the sectional view of a seal attachment region for the data pad side 40a taken along line II-II′ in FIG. 3. FIG. 4C shows the sectional view of the seal attachment region for the dummy seals 37 taken along line III-III′ in FIG. 3. The liquid crystal injection port side 40b and the open side 40c have a similar cross-section to FIG. 4C.
As shown in FIGS. 4A, 4B and 4C, the black matrix 14 and the common electrode 15 of the upper substrate 11 are formed on the glass substrate 33b uniformly over every region of the data pad side 40a, liquid crystal injection port side 40b, open side 40c, gate pad side 40d and the dummy seals 37. On the lower substrate 17 having the thin film transistor array thereon, seals 31 are formed over the gate pad side 40d, as shown in FIG. 4A, and formed over the data pad side 40a, as shown in FIG. 4B. The dummy seals 37 are formed, as shown in FIG. 4C, having a slightly larger thickness than the seals 31.
A gate insulation layer 42 made of silicon nitride film (SiNx) and a passivation film 43 are sequentially stacked on the gate electrode layer 41, which is formed on the glass substrate 33a, at gate pad side 40d. The gate electrode layer 41 is formed to prevent static electricity that can be generated due to frequent movement of the substrates during fabrication of the liquid crystal display device. The total thickness of patterns formed on the lower substrate 17 at gate pad side 40d is about 8500 angstroms, for example, if an inorganic passivation film is used.
The seal 31 at the data pad side 40a is formed above a patterned active layer 45 formed on a gate insulation layer 42 made of silicon nitride (SiNx), a source/drain electrode 46 is formed on the active layer 45, and a passivation film 43 is formed on the source/drain electrode 46. The total thickness of patterns formed on the lower substrate 17 at the data pad side 40a is about 9500 angstroms, for example, if an inorganic passivation film is used. The dummy seals and seals 31 at the liquid crystal injection port side 40b and open side 40c are formed at regions of the substrate having a passivation film 43 formed on the gate insulation layer 42. The total thickness of patterns formed on the lower substrate 17 at regions in between the panels where dummy seals are formed is about 6000 angstroms, for example, if an inorganic passivation film is used.
As described above, the thickness of the patterns formed on the lower substrate differs according to the attachment regions of the lower substrate. Thus, if a seal with a uniform thickness is formed for attaching the upper and lower substrates, the liquid crystal cell gap will not be uniform across an entire panel of the substrate. Accordingly, optical characteristics of the panel will be skewed or incorrect in portions of a panel where the liquid crystal cell gap is not uniform. In addition, when a large-scale upper substrate 11 and lower substrate 17 are attached, the capability of exhausting air between the substrates is degraded by long narrow passages, such as shown in the section (IV-IV′) in shown in FIG. 5, between the dummy seals. Furthermore, the dummy seals 37 may be broken down by air exhaust pressure.